The answer depends on the size of the habitat and how it is set up.

Plenty of room, however I would add more than 2 ghost shrimp as they do better in larger groups. Try out 4-5 of them.

That wont stop them from laying eggs all over everything

If the male is building a nest it’s a sign that they are healthy enough to mate. When you are healthy you are generally happy right? That’s the correlation there but you are correct in that activity level can be a sign of health and happiness too. The caveat here is when the water is too cold from not being heated or as a betta gets older they will become more lethargic in their activity levels.

Building a bubble nest doesn’t necessarily mean they are happy they do it for breeding. A good way to tell if they are happy is by how much they swim. If they swim often but still rest and hide thats normal and happy behavior. If they don’t swim often like the ones in the little pet store cups that’s not good.

I had two zebra danio fish and one betta in my first 5 gallon aquarium and they got along just fine and still do in my 35 gallon aquarium. I just added 3 koi fish today and monitoring them to see if they will also co exist.

Males and females should not be housed together